Northern Spain in June / need for reservations

In early June, we are bregining a trip with 3 nights Barcelona and have reserved a hotel. The plan is to drive west trought the pyrenees with stop offs at interesting locations along the way. Then on to Pampoloma, San Sabastiian Bilboa and Santander and on to Madrid and home. The question is how criticial is it to have prearranged reservations along the way? I think Madrid makes sence but what about the other stops and in the mountains in Early June?
Thanks.

'The plan is to drive west trought the pyrenees' You know you can't really do that right, since there are no roads running east-west? Drive along the bases (either side) and, maybe, make a bunch of spurs into the mountains, but that's going to take seven evers. However you do it, you won't need reservations along the way. Think real hard about why you're going to Santander.